1. Field of the Invention
The present invention relates to an electrophoretic display device configured to implement a display based on migration of electrophoretic particles.
2. Related Background Art
In recent years, amounts of data of various information are increasing steadily with development of information devices and outputs of information are provided in various forms. In general, the outputs of information are broadly classified under display on display monitors such as cathode ray tubes, liquid crystal display devices, and so on; and hard copy display on paper by means of printers or the like.
Concerning the display on display monitors, there are increasing needs for low-power-consumption and low-profile display devices, among which the liquid crystal display devices have been and are actively being developed and commercially available as display devices that can meet such needs. The current liquid crystal display devices, however, have problems that letters on the screen are not always clearly viewed, depending upon angles of view to the screen or reflected light and that they exert strains on vision resulting from flicker of a light source, low luminance, etc., which are not solved adequately yet. On the other hand, the monitor display using the cathode ray tubes exhibits satisfactory contrast and luminance, as compared with the liquid crystal display, but it is hardly mentioned that the monitor display can assure adequate display quality, as compared with the hard copy display described hereinafter, because of occurrence of flicker and the like. Another drawback is extremely low portability, because the device is large and heavy.
On the other hand, the hard copy display has been considered to become unnecessary with progress in electronization of information, but it is the present status that a huge number of hard copy outputs are still used in practice. A reason for it is that, in addition to the foregoing problem concerning the display quality, the monitor display of information has the resolution of at most about 120 dpi in general, which is considerably lower than those of printouts on paper (normally, 300 dpi and higher). Therefore, the monitor display imposes greater fatigue on the sense of sight than the hard copy display. As a result, it is often the case that hard copy outputs are made once even if images can be viewed on the display device. Further, the hard-copied information is free of the restriction on the display area to the size of the display device as imposed on the monitor display, many hard copies can be placed in favorable arrangement, hard copies can be rearranged without complicated device operation, and they can be checked one by one, which are also significant reasons for the combinational use of the hard copy display even in the case of the monitor display being available. Moreover, the hard copy display requires no energy for retaining the display, and secures excellent portability of whenever and wherever the information can be checked as long as the volume of information is not extremely large.
As far as display of dynamic picture, frequent rewriting, or the like is not demanded, the hard copy display has the various advantages different from those of the monitor display, but also has the drawback of heavy consumption of paper. Active development is thus under way in recent years to develop rewritable recording media (recording media permitting multiple recording and erasing cycles of images with high visibility and requiring no energy for retaining the display). The third rewritable display method succeeding to the properties of the hard copy display as described above will be called paperlike display.
Necessary conditions for the paperlike display include rewritability, no or adequately low energy necessary for retention of display (memory performance), excellent portability, excellent display quality, and so on. At present, display schemes that can be regarded as the paperlike display, include, for example, reversible display media employing an organic low molecular weight-high molecular weight resin matrix system to effect recording and erasing by a thermal printer head (e.g., Japanese Patent Application Laid-Open Nos. 55-154198 and 57-82086). This system is utilized as display part of some prepaid cards, but it still has such issues that contrast is not so high, that the number of recording and erasing repetitions is relatively small, approximately 150 to 500 cycles, and so on.
Another known display method available as the paperlike display is the electrophoretic display device invented by Harold D. Lees et al. (U.S. Pat. No. 3,612,758). In addition, Japanese Patent Application Laid-Open No. 9-185087 also discloses the electrophoretic display device.
This display device is comprised of a dispersion system having colored electrophoretic particles dispersed in an insulating liquid, and a pair of electrodes opposed to each other with the dispersion system in between. When a voltage is applied through the electrodes to the dispersion system, the electrophoresis of the colored particles causes the colored particles to migrate under the Coulomb force toward the electrode having the opposite polarity to that of the charge of the particles themselves and finally be adsorbed thereto. The display is implemented by making use of the difference between the color of the colored particles and the color of the dyed insulating liquid. Namely, an observer observes the color of the colored particles when the colored particles are adsorbed on the surface of the optically transparent, first electrode closer to the observer; on the contrary, the observer observes the color of the insulating liquid dyed so as to have different optical characteristics from those of the colored particles when the particles are adsorbed on the surface of the second electrode on the far side from the observer.
In the electrophoretic device of this structure, however, it was necessary to mix a coloring material such as a dye, ions, etc. in the insulating liquid. The existence of this coloring material caused new charge exchange and the material tended to act as an unstable factor in the electrophoretic operation, which sometimes resulted in degradation of performance, life, and stability of the display device.
In order to solve this problem, Japanese Patent Application Laid-Open Nos. 49-024695 and 11-202804 suggested the display device in which the electrode pair consisting of the first display electrode and second display electrode was placed on the same substrate and in which the colored electrophoretic particles were moved horizontally in the observer""s view. By making use of the electrophoretic property, the colored particles are made to migrate horizontally in a transparent insulating liquid between the first display electrode surface and the second electrode surface under application of voltage, thereby implementing the display.
In the electrophoretic display device of the horizontal migration type, the insulating liquid is transparent, the first display electrode and second display electrode have different colors in the observer""s view, and either one color is coincident with the color of the electrophoretic particles. For example, let us suppose the color of the first display electrode is black, the color of the second display electrode white, and the color of the electrophoretic particles black. Then, when the electrophoretic particles are distributed over the first display electrode, the second display electrode is exposed to exhibit white. When the electrophoretic particles are distributed over the second display electrode, the device exhibits black being the color of the electrophoretic particles.
The chemically stable electrophoretic device is implemented in this way.
Meanwhile, an electrophoretic particle is generally subject to a force acting along a direction of an electric field vector and being proportional to magnitude of the electric field vector, which appears between the display electrodes. It is thus ideally desirable that, at each pixel to be rewritten, an electric field of identical strength directed from the starting electrode to the destination electrode be exerted on all the electrophoretic particles over the starting electrode.
In the horizontal migration type electrophoretic display device, however, the magnitude of the electric field vector generated is strong in the peripheral regions of the display electrodes and weak in the central regions of the display electrodes. Further, the orientation of the electric field vector is horizontal in the border regions of the display electrodes and vertical in the central regions of the display electrodes. As a consequence, the absolute value of the horizontal component of the electric field vector becomes minimum in the central regions of the display electrodes. This state is schematically shown in FIG. 14. In the figure arrows represent forces exerted on the electrophoretic particles, directions of the arrows directions of the forces, and lengths of the arrows magnitudes of the forces. As a result, a nonuniform electric field is exerted on the electrophoretic particles over each display electrode in each pixel to be rewritten, and the electric field inappropriate for migration is exerted on the electrophoretic particles in the central region of the starting display electrode, as compared with the electrophoretic particles in the peripheral region of the starting display electrode.
For this reason, the electrophoretic particles in the central region of the display electrode take a longer time for migration to the desired electrode, so that the time can be a hindrance to reduction of drive time. Another possibility is that the particles fail to migrate to the desired electrode and remain existing over the starting electrode, so as to cause degradation of contrast.
The inventor has conducted the analysis and intensive research on the problem as described above and found that the above problem was able to be solved by modifying the display electrodes into adequate shapes and by providing an additional electrode structure on the display electrodes.
It is, therefore, an object of the invention to provide a horizontal migration type electrophoretic display device that permits electrophoretic particles to migrate to a destination display electrode without remaining over a starting display electrode, so as to achieve excellent display contrast, decrease the response time for display, and reduce a drive voltage without decrease in display contrast, solving the problem in the prior art described above.
Namely, according to the present invention, there is provided an electrophoretic display device comprising a first substrate, at least two display electrodes of a first display electrode and a second display electrode placed on the first substrate, a second substrate placed in opposition to the first substrate, means for applying a desired voltage to each electrode, a transparent insulating liquid filling a space between the first substrate and the second substrate, and a plurality of colored electrophoretic particles dispersed in the transparent insulating liquid, in which the colored particles are made to migrate between the first display electrode and the second display electrode, thereby effecting switching of display, wherein on at least one of the first display electrode and the second display electrode, an auxiliary display electrode is placed at a portion including at least a portion at which an absolute value of a horizontal component of an electric field vector generated over the first display electrode and the second display electrode upon application of the voltages to the first display electrode and the second display electrode, becomes minimum.
A characteristic configuration of the present invention is that the auxiliary display electrode is electrically connected to at least one of the first display electrode and the second display electrode located below the auxiliary display electrode.
Another characteristic configuration of the present invention is that the auxiliary display electrode is a projecting structure provided on at least one of the first display electrode and the second display electrode.
Another characteristic configuration of the present invention is that the auxiliary display electrode is an electrode placed through an insulating layer on at least one of the first display electrode and the second display electrode.
Another characteristic configuration of the present invention is that the auxiliary display electrode is placed in a region including a pixel peripheral portion on at least one of the first display electrode and the second display electrode.